For those hard of hearing, the comprehensibility of speech in an auditory environment with background noises represents a large problem. An optimization of the speech comprehensibility can be achieved via a skilled adjustment of the time- and frequency-dependent amplification of an acoustic input signal by a hearing aid device. The problem is thereby posed to find a rule or, respectively, an algorithm that can determine an optimal time- and frequency-dependent amplification for arbitrary (particularly also time-variant) speech signals/background noise mixes.
Given the use of the hearing aid device, it has so far been attempted to improve the speech comprehensibility via a skilled selection of setting parameters that influence the signal processing in the hearing aid device. For this, different, non-linear adaptation formulas have been used with different objectives. An adaptation formula designated under the designation “DSL i/o” substantially aims to restore the loudness of a person of normal hearing.
In contrast to this, given the adaptation formula known as “NAL-NL1”, the goal is pursued to achieve a maximum speech comprehensibility. While the loudness can be restored independent of the type of the noise, the speech comprehensibility depends on the spectral energy distribution of the speech signal relative to the spectral energy distribution of the noise signal. However, in NAL-NL1 this dependency on the background noise spectrum or, respectively, background noise level, remains unaccounted for since this adaptation formula assumes an optimization of the speech comprehensibility in a quiet situation. A further restriction in the cited formulas is that they are based on a static characteristic line, i.e., that the optimal loudness or, respectively, speech comprehensibility can not be achieved at any arbitrary point in time via the corresponding adaptation formula, but rather only in the temporal median.
Given the adaptation formula NAL-NL1, the optimal amplification for a specific hearing loss and a specific input signal is determined in advance (“offline”) and not in the hearing aid device. For this, that amplitude leading to an optimized speech comprehensibility is determined for each hearing loss/input level combination for the average speech spectrum with a speech comprehensibility and loudness model adapted to the hearing loss, without exceeding (in the feed of the hearing aid device) the volume that would be experienced in this situation by a person of normal hearing.
It is also known to provide adaptive filters for relief of background noise. Without consideration of the hearing loss, the amplification is calculated based on static assumptions about the speech signal and the noise signal, such that the speech signal is reconstructed from the speech signal/noise signal mix in a static sense, without taking into account perceptive effects in speech comprehensibility. Examples for this procedure are the use of a Vienna filter or spectral subtraction.